In recent years, from the viewpoint of global environment, for the purpose of improving the fuel economy of vehicles such as cars, magnesium alloys have been applied to the strength members forming engines, frames, and the like. Further, the magnesium alloys have been also widely applied as structural materials of casings of electric/electronic devices, engine components (piston, connecting rod), and the like of cars, aircraft, and the like.
For use as a structural material, magnesium (Mg) has a specific gravity of 1.8, and is practically the lightest metal (with a specific gravity about ⅔ that of aluminum, and about ¼ that of iron). Further, Mg is also excellent in specific strength, specific stiffness, and thermal conductivity.
However, when a magnesium alloy is used as a structural material of vehicles and the like for use under a high-temperature atmosphere, particularly when used as a member forming an engine, the magnesium alloy is exposed to temperatures as high as 200 to 300° C. For this reason, a heat resistance within this temperature range (high-temperature strength) is required thereof.
Conventionally, there have been developed various alloys obtained by improving the creep strength of a magnesium alloy. For example, there are known heat-resistant alloys obtained by adding elements such as silicon (Si), calcium (Ca), and rare earth elements to magnesium alloys containing prescribed amounts of aluminum, zinc, and the like, and other alloys (e.g., Patent Documents 1 and 2, and many others).
All of these magnesium alloys are intended to be improved in high-temperature strength by crystallizing or precipitating intermetallic compounds of the added elements and Mg into the grain boundary. These intermetallic compound phases include Al, Si, rare earth elements, Ca, and the like, and each have a high melting point This hinders crystal grains from sliding (grainsliding) under load-bearing condition at high temperatures, resulting in an improvement of the high-temperature strength.
On the other hand, in order to provide a heat-resistant magnesium alloy which is not reduced in bolt axial tension even when used under temperatures as high as 200° C., the following is also proposed: an alloy element is dissolved in solid solution in the magnesium matrix in order to prevent the reduction of the proof stress under high-temperature environment largely affecting the bolt axial tension (Patent Document 3). More specifically, the following is proposed: an alloy element having a radius larger than that of magnesium by a given amount, and having a maximum solubility in solid solution in magnesium of 2 mass % or more is added, and is dissolved in solid solution in an amount equal to or less than the maximum solubility in solid solution for intragrain strengthening.
Then, in Patent Document 3, as these elements, specifically, there are exemplified gadolinium (Gd), dysprosium (Dy), terbium (Tb), holmium (Ho) or yttrium (Y), samarium (Sm), and the like. Whereas, as comparative examples, Ca, Al, Zn, and the like are exemplified.
Further, a magnesium alloy is a difficult-to-work material, and hence, is unfavorably not easy to form into a desirable shape. Namely, the magnesium alloy is small in solidification latent heat, and high in solidification speed. For this reason, the magnetic alloy is difficult to cast, so that the resulting castings unfavorably tend to have defects such as cavities and elephant skin. Accordingly, for products whose appearance is regarded as important, the yield is low, and the defects must be subjected to a putty treatment, unfavorably resulting in a high cost. Further, the magnesium alloy is in a close packed hexagonal structure, and hence is low in ductility. Thus, working of a sheet material or a rod material thereof by pressing or forging is required to be performed at temperatures as high as 300 to 500° C. Even when working is performed at such high temperatures, there occur problems such as a low working speed, a larger number of steps, and a shorter die life.
In order to solve such problems of the difficulty in working of the magnesium alloy, the following method is proposed: in a step of continuously casting an AZ-based magnesium alloy having an aluminum content of 6.2 to 7.6 wt %, and thereby obtaining a billet, the mean crystal grain size of the billet is set at 200 μm or less by addition of a grain refiner and/or control of the cooling rate, and the resulting one is forged to manufacture a large-size component (see Patent Document 4). This document also describes the following: after working into the final product shape, a solution treatment and a T6 heat treatment are combined, thereby to set the mean crystal grain size at 50 μm or less, resulting in an enhancement of the corrosion resistance.
On the other hand, the following method is proposed: by means of a die casting or Thixo-molding forming machine, a magnesium alloy is formed into a sheet shape; the resulting sheet material is rolled at ordinary temperature to be applied with strain, and then is heated to 350 to 400° C.; as a result, the crystal is recrystallized, so that the crystal grain size is refined to 0.1 to 30 μm, resulting in an improved ductility (see Patent Document 5). The sheet material improved in ductility is formed by press working or forging.
Further, there are also shown methods in which a sheet material of a magnesium alloy is forged and formed, and by a plurality of steps of rough forging and finish forging, a boss with a height 7 times or 10 times or less the wall thickness of the formed product main part is formed (see Patent Documents 6 and 7).
However, for forming a component in a complicated and precise shape with a magnesium alloy, the method of forging from a billet as described in Patent Document 2 has its limit in terms of shape and wall thickness. On the other hand, with the method of forming from a sheet material of a magnesium alloy as described in Patent Documents 5, 6, and 7, production of a thin-walled component is possible. However, it is difficult to obtain a formed product in a complicated and precise shape by press working or forging of the sheet material.
In contrast, in recent years, also on a magnesium alloy, elucidation of the mechanism of expression of superplasticity has been pursued as with an aluminum alloy. This indicates the possibility of allowing working at a high strain rate by refinement of the crystal grain size (see, e.g., Non-Patent Document 1).    [Patent Document 1] JP-A-2004-238676    [Patent Document 2] JP-A-2004-238678    [Patent Document 3] JP-A-2003-129160    [Patent Document 4] JP-A-7-224344    [Patent Document 5] JP-A-2001-294966    [Patent Document 6] JP-A-2001-170734    [Patent Document 7] JP-A-2001-170736    [Non-Patent Document 1] p. 119 to 125, “Handbook of Advanced Magnesium Technology” edited by The Japan Magnesium Association